1. Field of Invention
The invention is directed to devices for milling a window in casing disposed in an oil or gas wellbore and, in particular, to casing window milling assemblies for cutting a window in the wellbore casing such as for allowing a lateral, offshoot, horizontal, or branch wellbore to be drilled.
2. Description of Art
Casing window milling assemblies for use with whipstocks disposed within wellbore casing are known in the art. These assemblies operate by lowering the assembly into a wellbore casing until a cutting end, or mill head, contacts the whipstock. As the assembly is further lowered, the mill head is forced into the wellbore casing by the whipstock. As a result, the mill head begins cutting the wellbore casing to form a window.
Contemporaneously, two additional, or secondary, mills such as a reaming mill and a honing mill begin cutting the wellbore casing above the window formed by the mill head. As the mill head moves further downhole, and is further forced into the wellbore casing by the whipstock, the opening being formed by the reaming mill and the honing mill gradually move toward the window formed by the mill head until the opening and the window connect. To assist with the bending moment caused by the mill head being forced by the whipstock into the wellbore casing, a flex-joint is usually disposed above the mill head.
As illustrated in FIG. 1, a typical prior art casing window milling assembly 10 contains three portions: mill head 12, flex-joint 14, and upper mill section 16. Upper mill section 16 includes secondary mills referred to as reaming mill 17 and honing mill 18. As mentioned above, flex-joint 14 allows assembly 10 to bend slightly as it is lowered down the wellbore casing (not shown) and mill head 12 contacts a whipstock (not shown) so that assembly 10 can be properly oriented with respect to the wellbore casing during milling operations.
Although these prior assemblies are effective at ultimately forming the desired opening in the wellbore casing, they have several shortcomings. For example, in casing window milling assemblies in which the reaming mill 17 and honing mill 18 are disposed toward the upper end of the assembly, i.e., toward the end opposite the end having the mill head 12, the reaming mill and honing mill routinely engage the wellbore casing prematurely causing the reaming mill and honing mill to lose their milling ability prematurely. As a result, the length of casing window above and near the top of the whipstock is reduced such that longer and larger diameter assemblies and other equipment which, in most cases, are more desirable, cannot pass through the opening.
Current casing window milling assemblies also experience problems with the mill head wearing out prematurely. As discussed above, the mill head cuts the wellbore casing at the same time the reaming mill and honing mill cut the casing. The three mills cutting at the same time excites severe vibration of the mills against the casing. This vibration impact can cause the cutters to breakdown and the mills loose their cutting ability prematurely. This can lead to the considerable expense of a second milling operation with a fresh set of mills.
Another shortcoming of the current window milling assemblies is that the mill head prematurely exits the casing producing a shorter window (more deviated wellpath) which may adversely affect the subsequent drilling or other wellbore operations. To cut a required length window it is essential that the mill head be kept in substantial contact with the whipstock until it nears the end of the whipstock ramp or scoop. The current milling assemblies attempt to achieve this by using the upper mill and high side of the casing contact as a fulcrum. But since either the upper mills and the casing are worn out or the casing is cut away as described in the preceding paragraphs, the constraint at the fulcrum is either relaxed or completely lost resulting in a side force which is often inadequate causing the mill head to drill away from the whipstock prematurely producing a reduced length window.
Accordingly, prior to the development of the present invention, there has been no casing window milling assembly or method of cutting an opening in a wellbore casing that: prevents the reaming mill and honing mill from prematurely engaging the wellbore casing; decreases the torque exerted on the casing window milling assembly; increases the life of the mills by decreasing vibration of the casing window milling assembly; and provides the side force need to prevent the mill head from drilling away from the whipstock before the desired opening length is achieved. Therefore, the art has sought a casing window milling assembly and a method of cutting an opening in a wellbore casing that: prevent the reaming mill and honing mill from prematurely engaging the wellbore casing; decrease the torque exerted on the casing window milling assembly; increase the life of the mills by decreasing vibration of the casing window milling assembly; and provide the side force need to prevent the mill head from drilling away from the whipstock before the desired opening length is achieved.